1. Field of the Invention
The present invention relates to a thermosensitive recording medium having an excellent light resistance, and an image processing method using the thermosensitive recording medium.
2. Description of the Related Art
Each image has been so far recorded and erased on a thermosensitive recording medium, (hereinafter, also referred to as “thermoreversible recording medium” or “recording medium”) by a contact method in which the recording medium is heated by making contact with a heat source. For the heat source, in the case of image recording, a thermal head is generally used, and in the case of image erasing, a heat roller, a ceramic heater or the like is generally used.
Such a contact image processing method has advantages in that when a thermoreversible recording medium is composed of a flexible material such as film and paper, an image can be uniformly recorded and erased by evenly pressing a heat source against the thermoreversible recording medium with use of a platen, and an image recording device and an image erasing device can be produced at cheap cost by using components of a conventional thermosensitive printer.
However, when a thermoreversible recording medium incorporates an RF-ID tag as described in Japanese Patent Application Laid-Open (JP-A) No. 2004-265247 and Japanese Patent (JP-B) No. 3998193, the thickness of the thermoreversible recording medium is thickened and the flexibility thereof is degraded. Therefore, to uniformly press a heat source against the thermoreversible recording medium, it needs a high-pressure. When irregularity is formed on a surface of the thermoreversible recording medium, it is difficult to record and erase an image using a thermal head or the like. In view of the fact that RF-ID tag enables reading and rewriting of memory information from some distance away therefrom in a non-contact manner, a demand arises for thermoreversible recording media as well. The demand is that an image be rewritten on such a thermoreversible recording medium from some distance away therefrom.
Consequently, a recording method using a laser in a non-contact manner is proposed as a method of recording and erasing each image on a thermoreversible recording medium from some distance away from the thermoreversible recording medium, or on a surface having irregularities of the thermoreversible recording medium (see JP-A No. 2000-136022).
An image recording and erasing method using a laser has been proposed, wherein a recording medium including a leuco dye, a reversible developer and various photothermal conversion materials in combination is used, and recording is performed thereon using a near infrared laser light (see JP-A Nos. 05-8537 and 11-151856).
However, the leuco dye has a problem that it is decomposed by light and oxygen, when exposed to sunlight the outdoors for a long period of time, and a background of a thermoreversible recording medium turns brownish. Moreover, there is another problem that when an image is recorded on the thermoreversible recording medium, and then is exposed to sunlight for a long period of time in the presence of oxygen, an image density thereof is decreased or the image cannot be completely erased however hard it is tried. Moreover, when the organic coloring matter having an absorption in the near infrared region, which is described in JP-A Nos. 05-8537 and 11-151856 is used as the photothermal conversion material, the organic coloring matter generally has low light resistance, and the decomposition of the organic coloring matter decreases the absorption in the near infrared region. Thus, the recording sensitivity and erasure sensitivity are significantly decreased.
To solve these problems, it has been attempted that an ultraviolet absorbing layer and an oxygen blocking layer are provided in a thermoreversible recording medium (JP-A Nos. 07-205547 and 09-175024).
Moreover, it has been proposed that a thermoreversible recording layer is sandwiched by a PET film having an oxygen permeability of 6 mL/(m2·24 hr·atm) at 20° C. and 60% RH and a resin film of an ethylene-vinyl alcohol copolymer having an oxygen permeability of 0.2 mL/(m2·24 hr·atm) at 20° C. and 60% RH (see JP-A No. 2004-160806). However, the resin film of an ethylene-vinyl alcohol copolymer is easily influenced by humidity, and the film absorbs moisture in a high humidity environment, such as the outdoors in summer, thereby increasing the oxygen permeability thereof, and oxygen cannot be sufficiently blocked. Thus, the resin film has insufficient light resistance and complete erasure cannot be performed.
Moreover, as disclosed in JP-A No. 2004-160806, in the case where, on the thermoreversible recording medium in which the photothermal conversion material is contained in a resin having a low heat resistance, such as an adhesive layer, image recording and erasing are repeatedly performed using a laser, the layer containing the photothermal conversion material is repeatedly heated at high temperature, and the resin component around the photothermal conversion material and the photothermal conversion material itself may be decomposed into gas. The generated gas is not let off from the thermoreversible recording medium and air bubbles are formed on the surface thereof, causing undesirable appearance, because the oxygen blocking layer is provided in the thermoreversible recording medium. This is a severe problem for practical use. This problem is not described in JP-A No. 2004-160806, and the inventors of the present invention has found for the first time. In view of this point, the improvement of the light resistance and durability against repeated use of the thermoreversible recording medium has been demanded at present.